What could cause the fan on my Kenmore microwave to sometimes not come on?

I have a Kenmore microwave model #665.61659101 serial number TRM0110080. It is not working properly. The food is being heated, but the fan is not coming on all the time. Sometimes it comes on right away, sometimes it waits 30 seconds or so before coming on. The light, turntable, etc. all work, but it is very quiet. Too quiet for a microwave. Is this a common problem? What part may I need to fix it myself? Is it time for a new microwave? Thanks.